1. Field of Application
The following description relates generally to telecommunications systems and wireless communications systems.
2. Prior Art
Multipoint broadcasting in a wireless cellular network has the potential to increase the network capacity by many folds. In a traditional cellular network, the mobile users experience strong interference at cell edge from the signals of neighbor cells. Multipoint broadcasting coordinates multiple base stations to transmit the signals to a set of mobile users in such a way that the signal intended for a mobile user is enhanced while the interference is minimized. Multipoint broadcasting has been adopted in LTE-A (Long-Term Evolution of 3rd generation of wireless cellular network, Advanced) and is referred to as CoMP (Coordinated Multi-Point transmission).
FIG. 1 illustrates an example multipoint broadcast system. Elements 102, 104, and 106 denote base stations 1, 2, and 3, respectively, and elements 152, 154, and 156 denote mobile stations 1, 2, and 3, respectively. Backbone 100 connects base stations 1, 2, and 3 to allow high-speed information exchange among the base stations. In FIG. 1, base stations 1, 2, and 3 form a set of collaborating multipoint broadcasters. Mobile users 1, 2, and 3 form a set of recipients. Base stations 1, 2, and 3, and mobile users 1, 2, and 3 together form a multipoint-broadcast set. Base stations 1, 2, and 3 transmit the combinations of the signals intended for mobile users 1, 2, and 3. At each base station, the combination “weight” for each mobile user signal can be different. Through elaborate algorithms, at each mobile user the desired signal in the received signal is enhanced whereas the undesired signal or the interference is cancelled or minimized. For example, when the transmitted signals from base stations 1, 2, and 3 arrive at mobile user 1, the signals for mobile users 2 and 3 are cancelled out or minimized, while the signal for mobile user 1 is maximized or enhanced, thus the signal quality of mobile user 1 improves significantly. Similarly, mobile users 2 and 3 will also see significant improvement in the quality of their respective signals. The combining of the different mobile-user signals at each base station is commonly referred to as “pre-coding”. The combining weight for each mobile-user signal at for each base station is an element in a so called “pre-coding matrix”.
Multipoint broadcasting requires the knowledge of the downlink channel (from base stations to mobile users) to be available at collaborating base stations for pre-coding. In a TDD (time-division duplex) network, base stations can obtain the downlink-channel information from the uplink-channel information by channel reciprocity, which eliminates the need for feedback as is required in an FDD (frequency-division duplex) network.
Ideal channel reciprocity, however, exists only between the antennas of base stations and the mobile users. The transmitter (TX) and the receiver (RX) of a base station are generally different from those of a mobile user or other base stations. This introduces the differences in gains and phases between downlink and uplink channels, and causes the non-reciprocity of the channel. To restore the downlink and uplink channel reciprocity, the differences in gains and phases need to be known. In general, the gain differences and the phase differences can be acquired and compensated separately and differently. It is relatively straightforward to calibrate the gains. On the other hand, phase differences are more critical in multipoint broadcasting, and are more difficult to estimate. When considering the acquisition of phase differences, the gains can be assumed to have been perfectly matched.
FIG. 2 presents a basic model for the non-reciprocity of the channel caused by the phase difference between a base station and a mobile user. Ignoring the gain mismatch, which can be calibrated and compensated separately as mentioned earlier, the TX/RX mismatches can be succinctly characterized by two random phases, carrier phase φb of a base station, and the carrier phase φm of a mobile user. The simple model in FIG. 2 can be shown to cover more elaborate models for TX/RX mismatches. Specifically, it is possible to reduce a more elaborate model to the basic model in FIG. 2 by establishing equivalent carrier phases φb and φm from the many parameters of the more elaborate model.
From FIG. 2, the overall downlink channel hdl, the overall uplink channel hul and the wireless channel h between the base station antenna and the mobile user antenna are related as follows:hd1=ej(φb−φm)h hul=ej(φm−φb)h  (1)Note that for a narrowband signal or a channel with flat fading, h is a complex scalar. For a wideband signal under a frequency-selective channel, h, φb, and φm can be functions of the frequency. From Eq. (1), the overall downlink channel hdl and the overall uplink channel hul are related byhdl=ej2(φb−φm)hul=ej2Δφhul  (2)where Δφ=φb−φm is referred to as the carrier-phase difference (CPD). When the context is clear, the overall downlink channel and the overall uplink channel will hereinafter be referred to simply as the downlink channel and uplink channel, respectively.
Eqs. (1) and (2) can be extended to the multipoint-broadcast set in FIG. 1 as follows. For downlink channel:
                                                                        [                                                                                                    h                        11                        dl                                                                                                            h                        12                        dl                                                                                                            h                        13                        dl                                                                                                                                                h                        21                        dl                                                                                                            h                        22                        dl                                                                                                            h                        23                        dl                                                                                                                                                h                        31                        dl                                                                                                            h                        32                        dl                                                                                                            h                        33                        dl                                                                                            ]                            =                            ⁢                                                [                                                                                                              ⅇ                                                      -                                                          jϕ                              1                              m                                                                                                                                                  0                                                                    0                                                                                                            0                                                                                              ⅇ                                                      -                                                          jϕ                              2                              m                                                                                                                                                  0                                                                                                            0                                                                    0                                                                                              ⅇ                                                      -                                                          jϕ                              3                              m                                                                                                                                                            ]                                ⁡                                  [                                                                                                              h                          11                                                                                                                      h                          12                                                                                                                      h                          13                                                                                                                                                              h                          21                                                                                                                      h                          22                                                                                                                      h                          23                                                                                                                                                              h                          31                                                                                                                      h                          32                                                                                                                      h                          33                                                                                                      ]                                                                                                                      ⁢                              [                                                                                                    ⅇ                                                  jϕ                          1                          b                                                                                                            0                                                              0                                                                                                  0                                                                                      ⅇ                                                  jϕ                          2                          b                                                                                                            0                                                                                                  0                                                              0                                                                                      ⅇ                                                  jϕ                          3                          b                                                                                                                    ]                                                                                        =                            ⁢                              [                                                                                                                              h                          11                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  1                                  b                                                                -                                                                  ϕ                                  1                                  m                                                                                            )                                                                                                                                                                                                                    h                          12                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  2                                  b                                                                -                                                                  ϕ                                  1                                  m                                                                                            )                                                                                                                                                                                                                    h                          13                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  3                                  b                                                                -                                                                  ϕ                                  1                                  m                                                                                            )                                                                                                                                                                                                                                                        h                          21                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  1                                  b                                                                -                                                                  ϕ                                  2                                  m                                                                                            )                                                                                                                                                                                                                    h                          22                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  2                                  b                                                                -                                                                  ϕ                                  2                                  m                                                                                            )                                                                                                                                                                                                                    h                          23                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  3                                  b                                                                -                                                                  ϕ                                  2                                  m                                                                                            )                                                                                                                                                                                                                                                        h                          31                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  1                                  b                                                                -                                                                  ϕ                                  3                                  m                                                                                            )                                                                                                                                                                                                                    h                          32                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  2                                  b                                                                -                                                                  ϕ                                  3                                  m                                                                                            )                                                                                                                                                                                                                    h                          33                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  3                                  b                                                                -                                                                  ϕ                                  3                                  m                                                                                            )                                                                                                                                                                          ]                                                                        (        3        )            And for uplink channel:
                                                                        [                                                                                                    h                        11                        ul                                                                                                            h                        12                        ul                                                                                                            h                        13                        ul                                                                                                                                                h                        21                        ul                                                                                                            h                        22                        ul                                                                                                            h                        23                        ul                                                                                                                                                h                        31                        ul                                                                                                            h                        32                        ul                                                                                                            h                        33                        ul                                                                                            ]                            =                            ⁢                                                [                                                                                                              ⅇ                                                      -                                                          jϕ                              1                              b                                                                                                                                                  0                                                                    0                                                                                                            0                                                                                              ⅇ                                                      -                                                          jϕ                              2                              b                                                                                                                                                  0                                                                                                            0                                                                    0                                                                                              ⅇ                                                      -                                                          jϕ                              3                              b                                                                                                                                                            ]                                ⁡                                  [                                                                                                              h                          11                                                                                                                      h                          21                                                                                                                      h                          31                                                                                                                                                              h                          12                                                                                                                      h                          22                                                                                                                      h                          32                                                                                                                                                              h                          13                                                                                                                      h                          23                                                                                                                      h                          33                                                                                                      ]                                                                                                                      ⁢                              [                                                                                                    ⅇ                                                  jϕ                          1                          m                                                                                                            0                                                              0                                                                                                  0                                                                                      ⅇ                                                  jϕ                          2                          m                                                                                                            0                                                                                                  0                                                              0                                                                                      ⅇ                                                  jϕ                          3                          m                                                                                                                    ]                                                                                        =                            ⁢                              [                                                                                                                              h                          11                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  1                                  m                                                                -                                                                  ϕ                                  1                                  b                                                                                            )                                                                                                                                                                                                                    h                          21                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  2                                  m                                                                -                                                                  ϕ                                  1                                  b                                                                                            )                                                                                                                                                                                                                    h                          31                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  3                                  m                                                                -                                                                  ϕ                                  1                                  b                                                                                            )                                                                                                                                                                                                                                                        h                          12                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  1                                  m                                                                -                                                                  ϕ                                  2                                  b                                                                                            )                                                                                                                                                                                                                    h                          22                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  2                                  m                                                                -                                                                  ϕ                                  2                                  b                                                                                            )                                                                                                                                                                                                                    h                          32                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  3                                  m                                                                -                                                                  ϕ                                  2                                  b                                                                                            )                                                                                                                                                                                                                                                        h                          13                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  1                                  m                                                                -                                                                  ϕ                                  3                                  b                                                                                            )                                                                                                                                                                                                                    h                          23                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  2                                  m                                                                -                                                                  ϕ                                  3                                  b                                                                                            )                                                                                                                                                                                                                    h                          33                                                ⁢                                                  ⅇ                                                      j                            ⁡                                                          (                                                                                                ϕ                                  3                                  m                                                                -                                                                  ϕ                                  3                                  b                                                                                            )                                                                                                                                                                          ]                                                                        (        4        )            The downlink channel and the uplink channel are related byhjidl=hijulej2(φib−φjm)=hijulej2Δφij  (5)where Δφij=φib−φjm is the CPD between base station i and mobile user j, hjidl is the over-all downlink channel from base station i to mobile user j, hji is the wireless channel between the antenna of base station i and the antenna of mobile user j, and hijul is the overall uplink channel from mobile user j to base station i. Note that Eqs. (3), (4), and (5) assume single antenna at each base station and mobile user. It is straightforward to extend Eqs. (3), (4), and (5) to include the general case of arbitrary number of antennas at each base station, arbitrary number of antennas at each mobile user, arbitrary number of base stations, and arbitrary number of mobile users.
From Eqs. (3), (4), and (5), it can be seen that base stations can obtain the downlink-channel information from the uplink-channel information and the CPDs. To this end, the base stations can acquire the uplink-channel information via uplink pilots, and acquire the CPDs by downlink-channel phase feedback from mobile users. This approach to acquiring the downlink-channel information, however, has following drawbacks:                1. For each multipoint-broadcast session, the CPDs have to be acquired first. This creates the CPD-acquisition overhead in time domain and reduces the network capacity.        2. The downlink-channel acquisition depends on the set of mobile users in a multipoint-broadcast session. A new multipoint-broadcast session with a different set of mobile users requires a new CPD acquisition, which further increases the CPD-acquisition overhead in time domain. If several multipoint-broadcast sessions are interleaved in time, i.e., time-multiplexed, this will increase the CPD acquisition overhead in time domain still further.        3. The CPD acquisition requires the feedback from all mobile users in a multipoint-broadcast session. As the number of mobile users in a multipoint-broadcast session increases, the demand on resources such as signaling or data channels for feedback also increases, which increases the feedback overhead, and/or the CPD-acquisition overhead in resource domain. The overhead in resource domain includes pilot sub-carriers in an OFDM (orthogonal frequency division multiple access) system, pilot code-channels in a CDMA (code-division multiple access) system, and data channels for feedback. Consequently the capacity of the network is reduced.        
The above drawbacks can result in large aggregated CPD-acquisition overhead and reduce the benefit of multipoint broadcasting.